In these days, electronic circuit boards have come to be used in various sorts of products, and the performance thereof has been improved day by day with increased frequencies used on the circuit boards. Flip chip mounting, which provides reduced impedance, is a mounting method suitable for electronic equipment that uses high frequencies. There is also demanded flip chip mounting for mounting on a circuit board an IC chip not in a package style but in a bear component style in compliance with the increase in number of portable devices. For this reason, when IC chips are mounted as a single chip on a circuit board or mounted on electronic equipment and a flat panel display, the IC chips include a certain amount of defective ones. Other than the flip chip, CSP (Chip Size Package), BGA (Ball Grid Array) and so on have been increasingly employed.
As a conventional method for bonding an IC chip onto the circuit board of electronic equipment (first prior art), there is the method disclosed in Japanese Examined Patent Publication No. 06-66355 and so on. This is shown in FIG. 15. As shown in FIG. 15, there is generally known the method of transferring an Ag paste 74 onto an IC chip 71 provided with bumps 73 for connection to electrodes 75 of a circuit board 76, thereafter hardening the Ag paste 74 and thereafter pouring an encapsulating material 78 between the IC chip 71 and the circuit board 76.
As a method for connecting an IC chip to a liquid crystal display (second prior art), as disclosed in Japanese Examined Patent Publication No. 62-6652 shown in FIG. 16A and FIG. 16B, there is generally known a semiconductor chip connection structure in which an anisotropic conductive film 80 is employed, and by peeling off an anisotropic conducting adhesive layer 81 constituted by adding conductive minute segments 82 into an insulating resin 83 from a separator 85 and applying the film onto a board or the glass of a liquid crystal display 84 and thermocompression-bonding an IC chip 86, the anisotropic conducting adhesive layer 81 is interposed between a lower surface of the IC chip 86 and the board 84 except for spaces under the Au bump 87.
As a third prior art, there is known a method for applying a UV-curing resin onto a board, hardening the resin located between the two by UV irradiation while mounting an IC chip on it with pressurization, and maintaining a contact between the two by the contracting force of the resin.
As described above, the IC chip bonding has been performed by performing the processes of die-bonding an IC chip of a flat package or the like onto a lead frame, connecting the electrodes of the IC chip to the lead frame by wire bonding, forming a package with resin encapsulation, thereafter printing a solder paste on the circuit board, mounting the flat package IC thereon, and subjecting the IC to reflow. According to the above-mentioned method called SMT (Surface Mount Technology), the process of packaging the IC is long and requires much time for the production of IC components, and it has been difficult to miniaturize the circuit board. For example, an IC chip encapsulated in a flat pack occupies an area about four to ten times greater than that of the IC chip, and this has been a factor of hindering the miniaturization.
In contrast to this, the flip chip method for mounting an IC chip in a bear chip style directly on a board for the reduction of process and the reduction in size and weight has come to be adopted lately. With regard to this flip chip method, there are developed many processing methods such as stud bump bonding (SBB) for performing bump formation on an IC chip, bump leveling, Ag.Pd paste transfer, mounting, inspection, encapsulation with encapsulation resin, and inspection and UV resin bonding for concurrently performing bump formation on an IC chip and UV-curing resin coating on a board and thereafter performing mounting, UV-curing of resin and inspection.
However, any of the processing methods has had the drawback that much time is required for the hardening of the paste for bonding the bumps of the IC chip to the electrodes of the board and for the coating and hardening of the encapsulation resin, leading to degraded productivity. There has also been the issue that ceramic or glass of which the quantity of warp is controlled is needed as a circuit board, leading to cost increase.
Moreover, according to the processing method that uses a conductivity paste as in the first prior art for the bonding material, it has been required to level and flatten the IC chip bumps before use in order to stabilize the quantity of transfer.
Moreover, in the bonding structure with an anisotropic conducting adhesive as in the second prior art, there has been developed one that employs glass as a board material of the circuit board. It is required to hold conductive particles between the IC chip side electrode and the board side electrode for electric continuity between the electrodes, and therefore, it is required to uniformly disperse the conductive particles in the conductive adhesive. However, it is difficult to uniformly disperse the conductive particles in the conductive adhesive, and this has disadvantageously caused short circuit due to the abnormal dispersion of the particles and led to the expensive cost of the conductive adhesive and the necessity of the formation of the IC chip bumps by electroplating in order to shape the height of the bumps.
Moreover, according to the method of performing bonding using a UV-curing resin as in the third prior art, it is required to set a bump height variation within a range of ±1 (μm), and there has been the issue that the bonding cannot be achieved in the case of a board of a degraded flatness such as a resin board (glass epoxy board). Moreover, according to the method using solder, it has been required to pour and harden the encapsulation resin in order to alleviate the thermal expansion and shrinkage difference between the board and the IC chip after bonding. A time of two to eight hours has been needed for the hardening of the encapsulation resin, and this has led to the issue that the productivity is extremely degraded.
Accordingly, the object of the present invention is to solve the aforementioned issues and provide an electronic component mounting method and apparatus for bonding electronic components to circuit boards with high productivity and high reliability by interposing an anisotropic conductive layer that has conductive particles needing neither a resin encapsulation process for pouring a resin between an electronic component and a board nor a bump leveling process for shaping the bump height after the electronic component is bonded to the circuit board as well as an electronic component unit obtained by mounting the electronic component on the board by the above-mentioned mounting method.